The present invention generally relates to an antiskid control device for a motor vehicle and more particularly, to an antiskid control device which is provided with a detection means for detecting malfunctioning of a wheel speed detecting means including a wheel speed sensor.
Conventionally, in wheel speed detecting means, it has been so arranged, for example, that after rotation of a gear fixed to a wheel has been detected as an AC signal by a magnetic pickup attached to a vehicle body, the AC signal is converted into a pulse train by an interface circuit such that either this signal of the pulse train is converted into a digital value indicative of the wheel speed by a time counting and counting circuit and an arithmetic circuit or the pulse train is transmitted to a F/V converter so as to be converted into a voltage corresponding to the wheel speed. A method is known in which when the pulse train or the digital value indicative of the wheel speed or the voltage corresponding the wheel speed cannot be suddenly obtained at all due to a malfunction in these circuits, a decision is made as to whether the malfunction is caused by locking of the wheels or disconnection, etc., by comparing a deceleration at that time with mechanically possible maximum deceleration based on, for example, inertia of the wheels.
However, in the known method, if the digital value indicative of the wheel speed or the voltage corresponding to the wheel speed exhibits, due to fracture of some of teeth of the gear or malfunctioning of the F/V converter, changes indistinguishable from those of a state of periodical repetition of locking of the wheels, it is decided as if locking of the wheels and recovery of the wheels from locking are repeated. Thus, in this case, pressure reduction and pressure increase are repeated endlessly in a brake device. Furthermore, known antiskid control devices usually increase pressure slowly also after completion of pressure reduction through repetition of, for example, pressure holding and pressure increase such that the wheels are not locked again immediately. As a result, in the known antiskid control devices, such a phenomenon may take place that even if a driver fully kicks a brake pedal, the brake is hardly applied.